Exogenous melatonin alleviates NO damage in tobacco leaves by promoting antioxidant defense, modulating redox homeostasis, and signal transduction.

Nitrogen dioxide (NO) is a common outdoor air pollutant, which has adverse effects on the environment and human health. Herein, NO inhibited photosynthesis and antioxidant capacity in plants. Melatonin (Mel) is a neurohormone found in the pineal gland. Exogenous Mel alleviated chlorophyll degradation and increased the expression of key proteins and genes in the process of chlorophyll synthesis in tobacco leaves exposed to NO. Additionally, the activities of photosystem II (PSII) and photosystem I (PSI) were enhanced. PSII and PSI reaction center proteins and genes were upregulated. Mel pre-treatment enhanced enzyme activities and expression of proteins related to the ascorbic acid-glutathione cycle and thioredoxin-peroxiredoxin pathway in leaves exposed to NO, thus regulating their redox balance. Furthermore, exogenous Mel mediated the polyamine synthesis pathway and increased the expression of the key enzyme proteins SAMS1, SAMS2, and SAMS3 in the polyamine synthesis pathway in leaves under NO stress. Mel regulated ABA signal transduction and calmodulin binding transcription factors CAMTA12 and NtCaM calmodulin NtCaM2 in Ca signal transduction. Collectively, these results elucidate that Mel can alleviate high-concentration NO, thus suitable for agricultural application.